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DC to DC charger: Does your RV need one?

What is a DC to DC charger and should my RV be equipped with one?

Before that question can be answered, you will need to understand what a DC to DC charger is and what functions it performs. Many of you reading this use an autoformer to assure the incoming shore power is at the proper voltage for your RV. As most of you know, low shore power voltage can damage the compressor in your air conditioning system and/or residential refrigerator, or cause a host of other problems. An autoformer adjusts, or what some say “conditions,” the voltage to the ideal level for your RV.REDARC DC to DC Charger A DC to DC charger functions much the same way as an autoformer by conditioning the 12 volts from your alternator to the ideal charging voltage for your house batteries.

What has recently changed that now requires a DC to DC charger on tow vehicles?

To meet mileage requirements, vehicle manufacturers began using “smart alternators” (aka variable voltage alternators) several years ago. In simplified terms, a smart alternator saves fuel by only supplying the immediate electrical needs for the vehicle. Reduced electrical load by the alternator equates to less mechanical load on the engine, which equals better fuel mileage. Unfortunately for RVers, a smart alternator is basically useless for charging auxiliary batteries in a travel trailer, fifth wheel or house portion of a motorhome. Even those with older “dumb” alternators can benefit from improved charge rates provided by a DC to DC charger.

Determining if your tow vehicle or motorhome has a smart alternator is rather easy. Click here to learn how.

Do I need one? Below are two scenarios to consider

1) If you hook up your RV, equipped with lead acid house batteries, to shore power every night, you probably won’t benefit from installing a DC to DC charger. Since the converter/charger in your RV keeps your house batteries charged while plugged in, your batteries will likely contain enough energy to supply your 12-volt needs while traveling between campsites. If you dry camp often or use a considerable amount of 12-volt power* while traveling between hookup campsites, you should definitely consider one. (*Running the slide outs in and out, residential refrigerator, 12-volt refrigerator, operating the RV furnace, using other appliances via an inverter, etc.)

Here’s why: A DC to DC charger installed on your RV acts like a 3-stage RV converter/charger powered by your alternator. It will condition or “step up” the voltage from the alternator to maximize battery charging, then step down and maintain the voltage as the house batteries reach full charge. This will charge your house batteries faster and more efficiently via the alternator when driving compared to not utilizing one.

May need to consider a charger

2) If you are considering installing lithium house batteries, especially in a motorhome, you will want to seriously consider a charger. Here’s why: Due to less internal resistance, lithium batteries can be charged and discharged much faster than conventional lead acid batteries.

A discharged lithium battery can demand more amperage than your alternator can readily supply. Overloading and damage to your alternator can occur when this happens. This is especially true when the house batteries are located in close proximity to the alternator, as you will likely find in a motorhome. This is due to less voltage drop over a shorter length of wire. Longer lengths of wire, like you would find between the alternator of a tow vehicle and the towable RV house batteries, typically provide enough resistance to protect the alternator.

This is another case where you will want to condition the 12 volts to an appropriate level. Just as a DC to DC charger can increase the charge rate to better charge your house batteries quicker, it can also limit the charge rate to protect an alternator when excessive power is demanded to charge lithium house batteries.

DC to DC Charger Installation is relatively easy

If you decide to install one, installation is relatively easy for do-it-yourselfers.

Hopefully, this short introduction to DC to DC chargers has helped you to understand what they are and how you might benefit from one.

They make a great Christmas gift for the RVer on your gift list!

Special thanks to REDARC for supplying a charger to the author for review and testing.

##RVT1081

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Bill T
1 month ago

I installed the Renogy 20 Amp DC to DC charger in our motorhome when I installed a lithium battery to replace the factory LA one. I also had to upgrade the onboard Progressive Dynamics multistage charger/converter to be compatible for the lithium battery charging requirements. Now we can camp all weekend long without the need to recharge and have the Renogy charger setup to switch on if we need to charge the battery while driving. This setup works great for our needs and is now easier to find nice car camping tent sites which are cheaper too. Installed it all myself, total cost for all the components $1800 CDN.

HT Morgan
1 month ago

I enjoyed the article and wondered if my question triggered it. My question was about installing a 24 volt dc air conditioner unit to the existing 12 volt battery system in my 2017 Cirrus truck camper? 
Can I just add a 12to24 dc2dc boost converter wired directly between the RV batteries and the air conditioner unit bypassing the existing electrical system?Question hasn’t been answered yet but it may have triggered the article. My search continues…

HT Morgan
1 month ago
Reply to  Dave Helgeson

Thanks, that’s what I was looking for. I think it’ll work for my application

Martin Stapleton
1 month ago

Every rig and personal preferences is different. I use a Victron dc to dc 30 amp charger. On our pull travel trailer there is 800 watts of solar on the roof with 600 amp hours of lithium-ion. Usually the solar array is enough to bring storage capacity up to 100%, whether we’re boondocking or on a land-line. But if we are dry camping, and there is either too much shade or too many continuous cloudy days, the dc to dc charger has proven invaluable. No need to have a generator and container of gas to take up storage and add weight.

Fred
1 month ago

There are two potential problems with using a dc to dc charger when you’re boondocking. They charge at a slower rate than if you’re plugged into shore power, & if you drive a diesel, there is a lot of controversy about idling a diesel for long periods. My diesel automatically shuts off after 30 minutes of idling. Plus, using a big, idling diesel engine as a generator, with today’s diesel prices may not be smart. A small generator will charge at a faster rate & a lot less fuel cost. I see some benefits for gas vehicles, but many rvers drive diesels.

Leonard
1 month ago

About a year ago I installed the RedArc 50 amp DC/DC charger. I put the connector under the hood, so when dry camping my truck is facing my 5th wheel when charging the batteries. I did not run a cable to the back as I would then have the exhaust of the truck under our bedroom when charging. I will probably run an additional cable to the back for charging the batteries between stops sometime this winter. Currently this has not been needed as we do not normally drive without shore power for more than 3 days. With 4 x AGM batteries hooked up, I can easily go without recharging for a few days. So much happier with this setup than lugging around a heavy and noisy generator, even though mine was a Honda which is industry best.
To each their own, but the RedArc has been nothing short of fantastic! It also is a solar charge controller!

David
1 month ago

You neglected to mention the benefits for those of us who tow an RV trailer. I installed a Renogy 20 amp DC-to-DC charger in our truck with a cable connection to the lithium battery in our trailer. Thus, when we travel, we always show up at our destination with a fully charged trailer battery. And, while camping, if our solar panels will not keep our trailer battery charged, due to cloudy weather, all we have to do is connect our truck to the trailer and run the engine for about 45 minutes. Our battery is then fully charged!

David
1 month ago
Reply to  Dave Helgeson

Thanks for the clarification, Dave. When I see the term “house batteries,” I usually think of a motorhome, not a trailer. So, your comments about trailers went right past me! 🙂

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